The present invention relates to data communications equipment, e.g., modems, and, more particularly, to modems synchronized to the network.
For a large number of users, the telephone local-loop is still the primary transmission medium between a user's data communications equipment, i.e., a modem, and a local central office, which is a part of the Public Switched Telephone Network (PSTN). The telephone local loop is an analog transmission medium in which modem transmission signals are restricted to the voiceband with a nominal bandwidth of 3.5 kilo-Hertz (kHz). With the increase in the number and different kinds of data-oriented services, like Prodigy,.RTM. available, and the simple accessing of information via the well-known Internet, users have shown the desire to increase the data transmission speeds over the telephone local loop beyond what is currently available in off-the-shelf modems.
In the co-pending, commonly assigned, United States patent applications of: Ayanoglu et al., entitled "A High-Speed Modem Synchronized to a Remote Codec," Ser. No. 07/963539, filed on Oct. 20, 1992; N. R. Dagdeviren, entitled "A Modem with Received Signals and Transmitted Signals Comprising Signal Sets," Ser. No. 08/080161, filed on Jun. 21, 1993; and Ayanoglu et al., entitled "High-Speed Quantization-Level-Sampling Modem with Equalization Arrangement," Ser. No. 08/176742, filed on Jan. 3, 1994; a high-speed modem technology is disclosed that significantly increases the data transmission speed over the telephone local loop. Specifically, these patent applications describe a high-speed modem technology in which a modem is synchronized both in time and quantization levels to the analog-to-digital (A/D) and digital-to-analog (D/A) converters, i.e., quantizing devices, of the PSTN. This synchronization effectively enables a subset of the quantization levels to be used as a signaling alphabet and thereby greatly reduces the quantization noise that is introduced by the PSTN on any transmitted data signals. As a result, data transmission speed is greatly increased. For example, this synchronization approach allows a modem to operate at the PSTN clock rate, and, under certain assumptions, e.g., no bandlimiting in the telephone local loops, a 64 kilo-bit per second (kb/s) data rate can theoretically be achieved over the telephone local loop. A modem using this high-speed modem technology is referred to herein as a "quantization-level-sampling"(QLS) modem, and the signals communicated in a QLS data communications system are referred to herein as "pulsed" signals.
As just mentioned, in order for the modulation scheme of the above-described QLS modem to work, there must be timing synchronization in both the transmit and receive directions between the QLS modem and a network sampling clock in the PSTN. This synchronization is necessary because the network sampling clock controls the sampling instants of any quantizing device located within the PSTN. For example, data symbols transmitted from a QLS modem to its local central office must reach the A/D converter of the local central office at the precise moment that the A/D converter reads each sample. Similarly, a receiving QLS modem must be synchronized to the network sampling clock of its local central office. Unfortunately, any deviations in timing, measured as timing jitter, introduce intersymbol interference into the respective received data signal. At these high data rates, the maximum allowable timing jitter in a QLS data communications system is typically very small. For example, the maximum allowable timing jitter for a data transmission rate greater than 42 kilobits/second (kb/s) can be less than 70 nano-seconds (ns).